feat: work on NMI interrupt support

https://bugzmanov.github.io/nes_ebook/chapter_6_2.html

Author: nathanleiby

src/core.rs

@@ -246,8 +246,7 @@ impl Cpu {
     {
         loop {
             if self.bus.poll_nmi_status() {
-                todo!("implement NMI interrupt");
-                // self.interrupt_nmi();
+                self.interrupt_nmi();
             }
 
             callback(self);
@@ -1016,6 +1015,17 @@ impl Cpu {
         self.lsr(mode);
         self.eor(mode);
     }
+
+    fn interrupt_nmi(&mut self) {
+        self.stack_push_u16(self.pc);
+        self.stack_push(self.status);
+
+        self.set_flag(Flag::Interrupt, true);
+
+        self.bus.tick(2);
+
+        self.pc = self.mem_read_u16(0xFFFA);
+    }
 }
 
 #[cfg(test)]

src/ppu.rs

@@ -69,10 +69,13 @@ impl Ppu {
         self.clock_cycles += cycles;
         // each scanline lasts for 341 PPU clock cycles
         let scanline = cycles / 341;
-        if self.scanline < 241 && scanline >= 241 && self
+        if self.scanline < 241
+            && scanline >= 241
+            && self
                 .registers
                 .control
-                .contains(ControlRegister::VBLANK_NMI_ENABLE) {
+                .contains(ControlRegister::VBLANK_NMI_ENABLE)
+        {
             // upon entering scanline 241, PPU triggers NMI interrupt
             self.nmi_interrupt = true;
             // The VBlank flag of the PPU is set at tick 1 (the second tick) of scanline 241
@@ -96,8 +99,19 @@ impl Ppu {
     }
 
     pub fn write_to_ctrl(&mut self, data: u8) {
-        // TODO: bugzmanov book sets ctrl.bits directly vs recreating .. I hit an error attempting that
-        self.registers.control = ControlRegister::from_bits_truncate(data);
+        let before = &self.registers.control;
+        let after = ControlRegister::from_bits_truncate(data);
+
+        // should we toggle an NMI interrupt?
+        let is_vblank_state = self.registers.status.contains(StatusRegister::VBLANK_FLAG);
+        let enabled_vblank_nmi = after.contains(ControlRegister::VBLANK_NMI_ENABLE)
+            && !before.contains(ControlRegister::VBLANK_NMI_ENABLE);
+        if is_vblank_state && enabled_vblank_nmi {
+            self.nmi_interrupt = true;
+        }
+
+        // update the register's value
+        self.registers.control = after;
     }
 
     pub fn write_to_mask(&mut self, data: u8) {